On an old Bally I would have thought it more likely to be a faulty coil, or if you are lucky a break in the wire near the lugs.  The hold coil is the fine wire.  Faulty EOS on a Bally means the coil will not fire (EOS no contact at rest, broken wire leading to EOS) or will blow the fuse (EOS never opens when flipper activates).

cheers
Ian

i would have to sort of agree with ballywannabe. if you had a faulty eos then the flipper would be very weak or go up very slow. It sounds like a coil or if you are lucky a broken wire off the coil. What is happening is the flipper is going up then the eos is opening but the hold up winding is not working, thus when the eos opens the flipper drops down and closes the eos and the flipper goes up again and so on.